A hydraulic machine, of the turbine, pump or turbine-pump type generally comprises a runner and a ring gate allowing control over the supply of water to the runner. To do that, the ring gate is able to move between an open position, in which it allows water to pass from a duct supplying the runner, and a closed position in which it closes the duct. The movement of such a ring gate is controlled, in the known way, by a plurality of hydraulic cylinders the rods of which are connected to the ring gate and movements of which are controlled by means of a valve. The movement of the rods in the cylinders therefore causes the movement of the ring gate in the duct supplying the runner. When the pistons of the cylinders, and therefore the rods, move in a first direction, the pistons lift the ring gate toward its open position and when they move in a second direction, the opposite of the first direction, the ring gate is lowered toward its closed position.
It will be readily understood that, for large hydraulic machinery, in which the ring gate is very heavy and bulky, movement toward the closed position requires significantly less force than movement toward the open position during which the ring gate is raised, because of the weight of this ring gate.
It is therefore necessary to supply the cylinders at a higher pressure when the pistons are being moved in the first direction than when they are being moved in the second direction.
The known control devices, which propose to solve this problem by allowing the supply to the cylinders to be switched between a first pressure and a second pressure, are complicated and require the addition of numerous components in addition to the control valve. These components require tricky settings and painstaking maintenance interventions.